Tighthead plastic drums are well accepted in the marketplace; they are used to contain and transport chemical, foodstuffs, and other liquids, both hazardous and non-hazardous. The drum ratings required for these products are dictated by DOT regulation and UN recommendations. The regulations mandate that a particular drum design must withstand breakage during certain drop tests, withstand a particular amount of deflection during loading tests and withstand leakage during pressure testing. Drop testing includes dropping a drum multiple times on its side and dropping the drum multiple times on its top corner from a height that depends on the desired drum rating. Pressure testing includes applying a hydrostatic pressure within the drum that depends on the desired drum rating. Further, in the United States, drums must be capable of being handled individually in order to be commercially successful, and therefore drums are commonly provided with handling rings.
Tighthead plastic drums are a commodity product. Performance specifications are well-known and current technology can produce a reliable product. Due to the commodity nature of these drums, minimizing cost against a relatively low fixed price ceiling is crucial to the profitability of the manufacturer. Because so much of the cost of producing a plastic drum is concentrated in the resin from which the drum is made, particular care must be taken in design and manufacturing to minimize the amount of resin used to make a drum that meets regulation requirements.
Tighthead plastic drums are shipped in ISO-containers and semi-trailers, among other modes. In these containers, sizing has been standardized so that quantity is maximized if the drums are sized properly with respect to their diameter. If not, shipping costs are increased, making the drums less commercially valuable.
Early embodiments of the tighthead plastic drums had detachable handling rings. Ordinarily, lifting devices such as parrot beaks squeeze such handling rings in order to lift and handle filled plastic drums. The drums can also be lifted and handled using other methods such as forklift tines. Eventual plastic technological advances allowed the handling ring to be constructed integrally with the drum as a single piece construction. Providing an integral handling ring avoids a heavy investment in molds and molding machines. During the molding process of integrally molded drums, the handling ring of the drum is compression molded, resulting in the formation of a weld line and also resulting in large quantities of excess material or extrudate being pushed into the interior of the drum below the weld line. Breakage of the drum often occurs in this weld area.
The tendency of a drum to break depends in part upon the location of the handling ring and the internal geometry of the drum, which to an extent is also a function of the location of the handling ring. The points of weakness where drums are generally susceptible to fracture generally occur at the transitions between sections of differing thickness, or at points where the vessel walls change direction. Further, in molding plastic it is best to avoid creating stress initiation points which can be formed in internal comers that are sharply angled or in areas with small radiuses. These stress initiation points especially render a drum susceptible to breakage when they are located in parts of the drum that are exposed to high levels of stress during an impact event.
In some cases drums are designed so that the intersection of the head and sidewall of the drum define the location of the handling ring and thus the location of the critical weld line and the extrudate material. In another design, disclosed in U.S. Pat. No. 5,033,639 (Przytulla) a handling ring is located at or integral with a point where the sidewall meets a frustoconical transition section between the head and the sidewall of the drum. A schematic representation of this configuration is shown in FIG. 11. FIG. 11 includes a representation showing where extrudate 7 is formed within the drum as a result of compression molding the handling ring.
The location of extrudate material, caused by the molding process, is dependent on the location of the handling ring and in this case the extrudate is in part disposed along the interior of the sidewall. The formation of extrudate material creates a heavy section at this location. Such heavy sections are generally less flexible than thinner sections because they are more crystalline, whereas the plastic in the thin sections is more amorphous. In the drum design of FIG. 11 the heavy section is disposed in an area of high stress concentration, and thus in an impact situation will cause a drum to fracture more easily. Further, the extrudate material may have been formed so that it has stress initiators in the form of internal corners with sharp angles formed therein. In this case, the stress initiators can especially render a drum more susceptible to breakage because they are positioned in an area that is subject to high stress concentration during an impact event.
Another example of an integrally molded handling ring is disclosed in U.S. Pat. No. 5,543,107 (Malik et al.). A schematic representation of the Malik et al. handling ring is shown in FIG. 12. Malik et al. discloses a handling ring that is located in direct contact with the flat head surface of the drum. As discussed above, this design increases the chance of breakage because the handling ring is located near a transition point of two sections of varying thickness. In addition, the handling ring is molded so that the extrudate material forms a geometry referred to as a double ogee 5. This geometry is intended to eliminate acute angles formed in the extrudate which could act as stress initiation points. The formation of the geometry disclosed in this patent, however, requires a complex molding process. Further, such a location of the handling ring as disclosed in Malik et al. requires a relatively thick head portion of the drum in order to get enough extrudate to form the double ogee. The formation of the ogee itself and the formation of the thick head section requires more material, which adds to the cost of manufacture of the drum. In addition this design disposes the extrudate at an area of high stress concentration which has the disadvantages associated with heavy sections and stress initiation points discussed above.
Accordingly, it is desirable to provide a handling ring monolithically molded to a plastic drum that is located in a position that results in a drum that is less susceptible to fracture and that affords an internal geometry that reduces the chance of drum breakage. Further, it is desirable to provide such a handling ring that is simple and cost efficient to manufacture.